It has become well known that T-wave alternans has predictive value for arrhythmic events such as tachyarrhythmias. T-wave alternans has been determined to be an indicator of various forms of disordered ventricular repolarization, including disorders found in patients with cardiomyopathy, mild to moderate heart failure, and congestive heart failure. The following literature references deal with the subject of T-wave alternans as a predictor: Klingenheben T, Siedow A, Credner S C, Gronefeld, et al., T-Wave Alternans in microwave frequency as a new indicator of disordered ventricular repolarization: pathophysiology, methodology, clinical results, Z Kardiol, 1999, Dec., 88 (12), 974-81; Klingenheben T, Zabel M, D'Agostino R B, Cohen R J et al., Predictive value of T-Wave Alternans for arrhythmic events in patients with congestive heart failure, Lancet, 2000, Aug. 19; 356(9230): 651-2; and Hennersdorf M G, Perings C, Niebch V, Vester E G, et. al., T-Wave Alternans as a risk predictor in patients with cardiomyopathy and mild-to-moderate heart failure, Pacing Clin Electrophysiol 2000 Sep.; 23(9); 1386-91.
T-wave alternans (TWA) may be caused by changes in ion exchange during repolarization. If there is a change in the repolarization mechanism on one beat, the heart attempts to readjust on the following beat. This is manifested as an alternating change in the action potential. In the surface ECG this is seen primarily as an amplitude change. For an implanted medical device such as a cardiac pacemaker, the intracardiac electrogram (iecg) also shows a change in timing. Thus, the term T-wave as used herein may refer to a portion of the ventricular QRS-T-wave complex that includes the T-wave and the QRS-T segment. The alternating feature of TWA can be detected by examination, for example, of the QT interval, T-wave width, T-wave morphology, etc. Whatever the designated portion of the iecg, T-wave alternans refers to an alternating pattern of the wave that can be designated “A-B-A-B-A . . . ” where A represents every other cycle and B represents every other alternate cycle. As discussed in the literature, when such an alternating pattern appears, the different rates or forms of repolarization of the ventricular cells are statistically associated with a variety abnormal cardiac conditions. Further, the alternating repolarization pattern can lead to increased instability and consequent cardiac arrhythmias. Thus, T-wave alternans is recognized as an indicator of risk for ventricular arrhythmia and even sudden cardiac death.
The prior art discloses several different methods and techniques for detecting T-wave alternans. TWA can be measured non-invasively by exercise-inducing an elevated heart rate in the patient and then measuring the surface ECG with special electrodes and computer analysis. Moreover, it has been disclosed that measurement of the TWA through the IECG obtained by an implanted medical device provides the capability of obtaining improved waveform data and analysis for detection of TWA. See U.S. patent application Ser. No. 09/558,871, filed Apr. 28, 2000, “Implantable Medical Device and Method Using Integrated T-Wave Alternans Analyzer”, Morris et al. This patent is incorporated herein by reference in its entirety.